***To fulfill the requirement i have included one subquery in the existing query mentioned below.The output i am getting but it is performing Cartesian join and here i want to avoid Cartesian join.
I need ur suggestion if in my new added subquery i am doing any mistakes.

AND request.requestid IN
(SELECT requestid
FROM requestassignmenthistory rh,
topic t
WHERE lower(t.topicdescription) LIKE lower('topic')
AND rh.topicid=t.topicid

There is no execution plan posted; what IS that plan? And are you now saying that the query you posted is NOT the query you're having issues with? If it is the 'problem' statement it will have a sql_id value you can use to return the execution plan.

Provide the execution plan (you DO have it because you are reporting a CARTESIAN JOIN and you wouldn't know that information if the plan wasn't available).